One important aspect of digital images are the facial expressions of subjects depicted in the digital image. For instance, facial expressions often convey information to a viewer, such as the subject's emotions or mood. Accordingly, in order to alter or modify the depicted emotion or mood, an editor may desire to edit the facial expression of one or more subjects of the image.
Facial expressions include a composite of different facial features, such as eyes, nose, mouth, and the like. When a subject varies their expression by varying the size, shape, or orientation of the features, the subject may intentionally or unintentionally couple the variations of multiple features. For instance, when a subject smiles, they may simultaneously vary the shape of their mouth and widely open their eyes. Furthermore, the relationship between the coupling of the variations of the multiple features may be complex, e.g. when smiling, a relationship between the variation of the shape of the mouth and the openness of the subject's eyes may be nonlinear.
Therefore, to realistically depict an alteration of the subject's emotions or moods when editing facial expressions, such complex coupling of the edited features may be required. Accordingly, to realistically alter a subject's emotions or moods, an editor may desire to generate complex and/or nonlinear coupling of the variations of multiple facial features.